In the 1990s, the practical application of an MR (Magneto-Resistive effect) head and a GMR (Giant Magneto-Resistive effect) head has contributed to the skyrocketing of the recording density and capacity of an HDD (Hard Disk Drive). However, since the problem of heat fluctuation of a magnetic recording medium became conspicuous in the early 2000s, the speed of the increase in recording density slowed down temporarily. Even so, a perpendicular magnetic recording was put in practical use in 2005, the perpendicular magnetic recording being more advantageous to high density recording theoretically than an longitudinal magnetic recording. This event triggers a recent growth rate of 40% for the recording density of an HDD.
According to a latest demonstration experiment for an HDD, 400 Gbits/inch2 has been attained. If this trend continues strongly, a recording density of 1 Tbits/inch2 is expected to be attained around 2012. However, it will not be easy to attain such a high recording density even employing the perpendicular magnetic recording, because the problems of heat fluctuation will still become conspicuous.
A “high-frequency magnetic field assist recording method” is proposed as a recording method which can solve this problem (U.S. Pat. No. 6,011,664). In the high-frequency magnetic field assist recording method, the magnetic field with a frequency sufficiently higher than a recording signal frequency near the resonant frequency of a magnetic recording medium is locally applied to the medium. As a result, the medium resonates, and a portion of the medium, to which the high frequency magnetic field is applied, has a coercive force half or less than that of the medium without any field applied thereto. According to this effect, it is possible to write into a magnetic recording medium with a higher coercive force and higher anisotropy energy (Ku) by superimposing the high frequency magnetic field onto the recording field thereof. However, the method employs a coil to generate the high frequency magnetic field, making it difficult to efficiently apply the high frequency magnetic field to the medium.
Consequently, a method employing a spin torque oscillator has been proposed (for example, USPA20050023938, USPA20050219771, USPA20080019040, IEEE Trans. On Magn., Vol. 42, No. 10, PP. 2670). In the method disclosed, the spin torque oscillator includes a spin injection layer, a nonmagnetic layer, a magnetic layer and electrodes. A direct current is passed through the spin torque oscillator via the electrodes to cause ferromagnetic resonance of magnetization in the magnetic layer, the ferromagnetic resonance being due to spin torque by spin injection. As a result, the spin torque oscillator generates the high frequency magnetic field.
Since the spin torque oscillator is about tens of nm in size, the high frequency magnetic field generated localizes in an area of about tens of nm near the spin torque oscillator. Furthermore, the in-plane component of the high frequency magnetic field allows it to cause the ferromagnetic resonance in a perpendicularly magnetized medium and to substantially reduce the coercive force of the medium. As a result, a high-density magnetic recording is performed only in a superimposed area of a recording field generated from a magnetic pole and the high frequency magnetic field generated from the spin torque oscillator. This allows it to use a medium with a high coercive force (Hc) and high anisotropy energy (Ku). For this reason, the problem of heat fluctuation can be avoided at the time of high density recording.
In order to make a recording head for the high-frequency magnetic field assist recording, it becomes important to design and produce the spin torque oscillator capable of providing a stable oscillation with a low driving current and generating an in-plane high-frequency magnetic field to sufficiently cause a magnetic resonance of the magnetization in the medium.
A maximum current density which can be passed through the spin torque oscillator is 2×108 A/cm2, for example when the oscillator is about 70 nm in size. The current density beyond this value deteriorates the characteristic of the spin torque oscillator, e.g., owing to heating and electromigration thereof. For this reason, it is important to design a spin torque oscillator capable of oscillating with a current density as low as possible.
On the other hand, the literature discloses a criterion to fully cause a magnetic resonance of the medium magnetization. That is, it is desirable to make the intensity of the in-plane high frequency magnetic field not less than 10% of the anisotropy field (Hk) of the medium (see, for example, TMRC B6 (2007), “Microwave Assisted Magnetic Recording (MAMR)”). In order to increase the intensity of the in-plane high frequency magnetic field, the following items are required:    1) Increasing the saturation magnetization of an oscillation layer;    2) Increasing the thickness of the oscillation layer; and    3) Increasing a deflection angle of a precession motion of the magnetization in the oscillation layer, the deflection angle being defined as an angle between the magnetization and an external magnetic field.However, all the items give rise to an increase in the driving current.
There exists trade-off between lowering the driving current density and increasing the intensity of the in-plane high frequency magnetic field. It is desirable to develop a spin torque oscillator capable of realizing a lower driving current density and the higher intensity of the in-plane high frequency magnetic field simultaneously.
USPA20050110004 discloses an example employing an FeCoAl alloy as a material of the free layer in a longitudinal magnetization memory with TMR. USPA20070063237 discloses an example employing a Heusler alloy. FeCoAl was also employed in a longitudinal magnetization CPP-GMR head (J. of Appl. Phys., Vol. 101, P. 093905 (2007)).
USPA20080137224 discloses an example employing CoFeB for a spin injection layer and an oscillation layer.